1. Field of the Invention
The invention relates to a power converter suitable for conversion of low voltage sources to more usable voltage levels.
2. Background of the Invention
Thermoelectric power generation offers a unique and attractive alternative to conventional battery power generation due to its waste heat energy harvesting capabilities. It is especially suited for many low power and portable applications such as supporting remote sensor operations. The thermoelectric generator's output voltage is directly proportional to the temperature difference between its two junctions. In many cases, a small output voltage with a large current is obtainable with the thermoelectric generator when the temperature difference is less than 20K. Since many practical applications require a higher voltage than is obtainable with the thermoelectric generator, a DC-DC step up converter is important for this unconventional power supply.
Although the theory of a DC-DC converter has been well established, the use of a low input voltage (less than 0.7V) to drive a DC-DC converter is still a research subject.
Conventionally, step-up DC-DC converters have switched the input direct current to an alternating current, i.e. a DC to AC conversion followed by AC-AC transformation to a stepped-up voltage, followed by rectification of the stepped up voltage. One challenge for a low voltage input converter has been to find a switching mechanism providing maximum operation efficiency. MOSFET's have been used as switching devices in most converters due to their low on-resistance and low required gate current, leading to high converter efficiency. However, the switching voltage required is usually high as compared to voltages produced by thermoelectric generators.
A self-starting set of twin converter circuits containing a main converter and a starter circuit converter has been used to transform low voltage DC inputs. In that work, a normally-on transistor was used to perform switching for a low voltage input, and an output of a starter converter was used to drive a MOSFET based main converter. While the circuitry operated at low efficiency at the beginning of the operation when the starter circuit was performing the initial conversion, the main converter offered a relatively high conversion efficiency once the starter provided adequate switching voltage for the main converter.
The development of low voltage input power supplies has been reported for example in the following reference articles, all of which are incorporated herein by reference in their entirety:    [1] J. W. Kimball, IEEE Power Electronics Letters, Vol. 2, No. 3, September 2004;    [2] J. M. Damaschke, IEEE Trans. Ind. Applicat. Vol. 33, pp. 1203-1207, September/October 1997;    [3] B. Shen, et al., IEEE Electron Device Letters, vol. 17, no. 7, pp. 372-374, July 1996; and    [4] W. F. Chow, “Principles of tunnel diode circuits”, J. Wiley and Sons, Inc., 1964.
Despite this work, realization of suitable power conversion solutions has not been without problems limiting power extraction from low voltage power sources.